Fluidized bed techniques have been used to granulate powdery substances in the past. Normally, a bed of powder to be granulated is provided within a closed container, which has a porous bottom. Pre-heated air is injected through the bottom of the container to fluidize the bed of powder. A dust filter positioned in front of an outlet in the upper portion of the container prevents the fluidized powder from being discharged from the container through the outlet. By spraying liquid into the container while the powder is fluidized, various particles constituting the powder will stick together to form numerous larger agglomerates or granulates. This agglomeration or granulation process de-dusts the powder and makes it easier to handle. More particularly, the liquid which is sprayed into the closed container contacts the individual particles causing their surfaces to be partially dissolved, whereby the particles stick to each other to form the granulates or agglomerates. The liquid should preferably wet as much of the surface area of the particles as possible. Thus, in such fluidized bed granulating processes, the introduction and even distribution of the liquid is especially critical.
One method of wetting particles to be granulated involves spraying the particles with a mixture of water and air. A disadvantage of this method involves the relatively large amount of compressed air required to atomize the water. Also, because such atomization produces water droplets of various different sizes, it is difficult, especially in a relatively large fluidized bed apparatus, to distribute the water over all of the particles. Thus, it takes a long time to wet all of the particles when they are being granulated by a mixture of water and air.
Another common method of granulating particles involves the injection of the required amount of liquid in the form of steam. This steam injection method, which employs steam at a temperature of about 150.degree. C., tends to produce local overheating and excessive wetting of the particles in the vicinity of the steam nozzles, thereby causing the formation of lumps in the granulated product. Also, condensation forms on the steam nozzles, causing the particles to stick to them in lumps. Furthermore, the steam jets have a very limited range. For instance, steam nozzles located deep down within the fluidized bed apparatus only have a range of a few centimeters. Steam nozzles higher in the fluidized bed apparatus tend to inject the steam jets in the direction of the filter arranged in the upper portion of the fluidized bed apparatus, thereby causing the filter to become wet and therefore impeding the flow of air through it. When the steam comes into direct contact with the housing of the fluidized bed apparatus, condensation forms on the housing. The condensation causes the particles to collect in lumps on the housing. To disintegrate the lumps, the steam and the particles can be combined and, after the steam has condensed onto the particles to partially dissolve them, the particles are flung onto a rotating baffle-plate which causes the granulation of the particles. After the granulated particles are flung away by the rotation of the baffle-plate, they are subsequently pre-dried in a flow of hot gases and then finally dried in the fluidized bed apparatus. Such a steam granulation process is very expensive and also time consuming, as well as difficult to control. In spite of the use of the rotating baffle-plate, the formation of lumps and excessive wetting are frequently encountered when using this method.